The described subject matter relates generally to ionic liquids and more specifically to cleaning and regeneration of ionic liquids used for electroplating.
Pure aluminum forms a protective oxide film when exposed to oxidants such as air or water, resulting in a passivated surface that is resistant to corrosion. Aluminum alloys, and in particular, and high strength aluminum alloys are prone to localized corrosion. Similar to aluminum cladding, a pure aluminum surface layer can be used to protect Al alloys from corrosion. Electroplating of aluminum works well with complex geometries that would be impossible to be cladded due to line-of-sight and other process challenges. The corrosion resistance of pure aluminum surfaces can be further enhanced by passivation using a conversion coating treatment. In addition to coating high-strength aluminum alloys, electroplated aluminum has also been considered as a leading candidate to replace cadmium coatings on structural steels.
It is extremely difficult to electrolytically deposit aluminum and certain other metals from acidic aqueous solutions. The electronegativity of water relative to aluminum and certain other metals can result in unwanted formation of hydrogen in the plating bath. Al electroplating in ionic liquids have been demonstrated to address these issues on a wide range of metallic substrates.
Substrate pre-treatment has been shown to improve adhesion of the aluminum coating layer. Pre-treatment generally includes degreasing, cleaning, and activation. For reactive substrates such as aluminum alloys, a sacrificial displacement layer such as a zinc immersion coating can be deposited onto cleaned surfaces of the substrate. This can be done to inhibit surface formation of aluminum oxides and prevent subsequent unintended electrodeposition of aluminum coating material on surface oxides versus the alloy substrate. A zinc immersion coating can be deposited via an exchange reaction between zincate ions and the reactive metallic substrates. Alternatively, a displacement layer can be fully or partially etched in the same plating bath to further adjust the bonding strength of the coating.
Partial dissolution of sacrificial immersion coatings can occur spontaneously in conventional aqueous acidic plating baths. However, a number of ionic liquids suitable for electroplating can be extremely water-sensitive. Thus it is desirable to electrolytically remove (e.g., etch) the sacrificial immersion coating in the same or similar ionic liquid plating bath to prevent water contamination. At the same time, the electrolytic etching can result in rapid accumulation of contaminants in the ionic liquid plating solution. These contaminants can include byproducts of the sacrificial displacement layer. Elevated contaminant levels in the ionic liquid can affect the electroplating process if not controlled.